A subway-borne chemical attack is one of those theoreticals that require the willful ignorance of regular passengers—for most of us, it’s just better not to think about it. Not so for the NYPD, which yesterday announced a plan to test how a chemical or radiological attack would spread through the city’s 200-odd miles of subway, by pumping an invisible gas through the system this summer.

The Subway-Surface Air Flow Exchange, or S-SAFE, has actually been in the making for over a year—that’s how long it takes to plan a fake airborne toxic event in a city of almost two million. Led by Paul Kalb, the principal investigator on the $3.4 million Department of Homeland Security-funded grant, the group will track the movement of small amounts of Perfluorocarbon tracers (or PFTs) through the five boroughs on three days in July. “The study will show us the worst case scenario,” he explained over the phone today. “It’ll be a close representation of how particles from a bioweapon or dirty bomb could move through the air.”

What, exactly, are PFTs? They’re a completely odorless, invisible, and non-toxic type of gas that happens to have incredible staying power, making it perfect for tracking purposes (it’s even been used to trace counterfeit money). It also has a high vapor pressure, so it can pass through fabric and objects-and it’s easy to detect, because it isn’t found in nature.

The Pentagon allowed a private firm providing food and water to U.S. troops in Afghanistan to overbill taxpayers $757 million and awarded the company no-bid contract extensions worth more than $4 billion over three years, according to the Pentagon’s chief internal watchdog and congressional investigators.

The deal represented one of the largest U.S. military contracts in Afghanistan. But the Defense Logistics Agency, which was overseeing the contract, failed repeatedly to verify that the contractor’s invoices were accurate, an official in the Defense Department inspector general’s office said. “This has to be one of the prime poster childs for a government contract spun out of control,” Rep. John Mica (R-Fla.), said last week.

Mica and other members of the House Oversight and Governmental Reform Subcommittee on National Security expressed outrage at a hearing last week about the Pentagon’s handling of the deal, especially two contract extensions awarded amid a dispute between the government and the company over as much as $1 billion.